The present invention relates to a contact supervising circuit for a passenger protective device, for example, an air bag device for a vehicle, for securing the normal operation of the passenger protective device, more particularly to a contact supervising circuit for a passenger protective device in which a contact of a monitoring switch for controlling a flow of a monitoring current is supervised.
Known is a passenger protective device for safely protecting passengers by operating an air bag device upon collision of a vehicle. Such a passenger protective device of the kind as described above is so arranged that when a vehicle collides, a collision sensor detects the collision and a detonator is ignited to thereby bulk an air bag device.
It is impossible to secure safety of passengers when a fault is generated in the passenger protective device, and therefore supervision is always made as to whether the passenger protective device can normally operate or not.
As shown in FIG. 1, the conventional passenger protective device is provided with an ignition switch IGSW, two collision sensors S1 and S2, and a detnator Rs disposed between the two collision sensors S1 and S2 connected in series to a DC power suource. The device further comprises monitoring resistors R1 and R2 connected in parallel to the two collision sensors S1 and S2, respectively.
Then, supervision is made as to whether a monitoring current flows or not through the monitoring resistors R1 and R2 when the ignition switch IGSW of the passenger protecrive device is turned on, so that the possibility of the normal operation of the passenger protective device is asserted when the monitoring current flows.
In the conventional passenger protective device, assertion of the normal operation thereof has been effected as described above. Such a device is disadvantageous in that there is a possibility that the monitoring current does not flow in case that chattering of the ignition switch IGSW occurs as if when vehicle collision actually occurs even if the monitoring current is supervised, further, it becomes impossible to supervise the monitoring current when the ignition switch IGSW is turned off when the vehicle is parked, for example.
Then, in order to eliminate the foregoing problems, the passenger protective device is directly connected to the DC power source Vcc instead of being connected through the ignition switch IGSW, as shown by a dotted line in FIG. 1.
In the case where the passenger protective device is directly connected to the DC power source Vcc, however, another problem has arisen as to cause a risk of overdischarge of the DC power source Vcc (that is, the battery) or deterioration of the detonator Rs because a little monitoring current is allowed to flow always through the monitoring resistors R1 and R2.
In order to solve the latter problem, there has been proposed an arrangement shown in FIG. 2, for example. In the device shown in FIG. 2, a monitoring switch Ms operated in association with the ignition switch IGSW is connected in series to at least one of the two monitoring resistors R1 and R2 so that the monitoring switch Ms is closed simultaneously when the ignition switch IGSW is closed.
In this arrangement, however, there is a further problem that the contact of the monitoring switch Ms must be kept being supervised because there is a risk of overdischarge of the battery or deterioration of the detonator when the monitoring switch Ms is kept closed due to a fault thereof or the like, similarly to the case where passenger protective device is directly connected to the DC power source Vcc, as described above.